lkml.org 
[lkml]   [2011]   [Aug]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 14/32] nohz/cpuset: Don't turn off the tick if rcu needs it
    On Wed, Aug 17, 2011 at 04:10:27AM +0200, Frederic Weisbecker wrote:
    > On Tue, Aug 16, 2011 at 01:13:42PM -0700, Paul E. McKenney wrote:
    > > On Mon, Aug 15, 2011 at 05:52:11PM +0200, Frederic Weisbecker wrote:
    > > > If RCU is waiting for the current CPU to complete a grace
    > > > period, don't turn off the tick. Unlike dynctik-idle, we
    > >
    > > s/dynctik/dyntick/ ;-)
    >
    > Heh! :)
    >
    > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    > > > index 99f9aa7..55a482a 100644
    > > > --- a/include/linux/rcupdate.h
    > > > +++ b/include/linux/rcupdate.h
    > > > @@ -133,6 +133,7 @@ static inline int rcu_preempt_depth(void)
    > > > extern void rcu_sched_qs(int cpu);
    > > > extern void rcu_bh_qs(int cpu);
    > > > extern void rcu_check_callbacks(int cpu, int user);
    > > > +extern int rcu_pending(int cpu);
    > > > struct notifier_block;
    > > >
    > > > #ifdef CONFIG_NO_HZ
    > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > > > index ba06207..0009bfc 100644
    > > > --- a/kernel/rcutree.c
    > > > +++ b/kernel/rcutree.c
    > > > @@ -205,7 +205,6 @@ int rcu_cpu_stall_suppress __read_mostly;
    > > > module_param(rcu_cpu_stall_suppress, int, 0644);
    > > >
    > > > static void force_quiescent_state(struct rcu_state *rsp, int relaxed);
    > > > -static int rcu_pending(int cpu);
    > > >
    > > > /*
    > > > * Return the number of RCU-sched batches processed thus far for debug & stats.
    > > > @@ -1729,7 +1728,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
    > > > * by the current CPU, returning 1 if so. This function is part of the
    > > > * RCU implementation; it is -not- an exported member of the RCU API.
    > > > */
    > > > -static int rcu_pending(int cpu)
    > > > +int rcu_pending(int cpu)
    > > > {
    > > > return __rcu_pending(&rcu_sched_state, &per_cpu(rcu_sched_data, cpu)) ||
    > > > __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu)) ||
    > > > diff --git a/kernel/sched.c b/kernel/sched.c
    > > > index 0e1aa4e..353a66f 100644
    > > > --- a/kernel/sched.c
    > > > +++ b/kernel/sched.c
    > > > @@ -2439,6 +2439,7 @@ DEFINE_PER_CPU(int, task_nohz_mode);
    > > > bool cpuset_nohz_can_stop_tick(void)
    > > > {
    > > > struct rq *rq;
    > > > + int cpu;
    > > >
    > > > rq = this_rq();
    > > >
    > > > @@ -2446,6 +2447,19 @@ bool cpuset_nohz_can_stop_tick(void)
    > > > if (rq->nr_running > 1)
    > > > return false;
    > > >
    > > > + cpu = smp_processor_id();
    > > > +
    > > > + /*
    > > > + * FIXME: will probably be removed soon as it's
    > > > + * already checked from tick_nohz_stop_sched_tick()
    > > > + */
    > > > + if (rcu_needs_cpu(cpu))
    > > > + return false;
    > > > +
    > > > + /* Is there a grace period to complete ? */
    > > > + if (rcu_pending(cpu))
    > >
    > > This is from a quiescent state for both RCU and RCU-bh, right?
    > > Or can their be RCU or RCU-bh read-side critical sections held
    > > across here? (It would be mildly bad if so.)
    >
    > Yeah this can happen. This is called from the timer interrupt
    > or from an IPI. We can be in any kind of rcu critical section.
    >
    > > But force_quiescent_state() will catch cases where RCU needs
    > > quiescent states from CPUs, so is this check really needed?
    >
    > Yeah we should receive IPIs from CPUs that need us. This can
    > be an optimization though. No need to run into a cycle of
    > on timers shutdown/restart if we can complete something
    > right away.

    Never mind... I was confusing this with rcu_needs_cpu(), which should not
    be called from within an RCU-sched or RCU-bh read-side critical section.

    It is plenty fine to call rcu_pending() from within RCU read-side
    critical sections.

    Thanx, Paul


    \
     
     \ /
      Last update: 2011-08-17 04:53    [W:0.036 / U:1.040 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site